The first version (1) returns an object with the parameters currently associated with the distribution object.
The second version (2) associates the parameters in object parm to the distribution object.

A student_t_distribution is defined by a single parameter: n (its degrees of freedom). An object of type param_type carries this information, but it is meant to be used only to construct or specify the parameters for a student_t_distribution object, not to inspect the individual parameters.